It is desirable to have a constant source of fuel for a fuel pump within a fuel delivery system. To this end, fuel system designers have mounted a fuel reservoir within the tank which either remains filled as the fuel level within the tank falls, or, as is more commonly done, it continuously replenished with fuel from the tank, either by routing a portion of pressurized fuel to a jet pump to entrain fuel from the tank to the reservoir, as disclosed in U.S. Pat. No. 869,225 (Nagata et al.), or by routing return fuel to the reservoir as in U.S. Pat. No. 4,694,857 (Harris), or a combination of these, as disclosed in U.S. Pat. No. 5,070,849 (Rich et al.) and U.S. Pat. No. 5,218,942 (Coha et al.). Fuel pumps in the aforementioned patents draw fuel directly from the reservoir during operation. In many designs, the fuel pump is located within the reservoir, thus contributing to an increase in fuel temperature of the fuel drawn into the fuel pump due to heat generated from operation of the fuel pump. In addition, fuel returned from the engine to the reservoir is typically at a substantially higher temperature than fuel within the fuel tank. Higher temperature fuel typically contains a higher fuel vapor content, all other factors being equal, which is undesirable from an engine performance standpoint.
One solution to the problem is to pressurize the fuel in the reservoir so as to reduce vaporization of fuel therein, as disclosed in U.S. Pat. No. 5,431,143 (Brown), assigned to the assignee of the present invention and incorporated by reference herein. An additional advantage of that invention is drawing fuel directly from the fuel tank, which is typically cooler than fuel in the reservoir, as described above.
A problem has been noted, however, with the foregoing invention. When the fuel system is operating in a high temperature environment, for example in desert or arid regions when fuel tank temperatures can reach 140.degree. F. or higher. It has been found that the pressure differential between the fuel reservoir and the conduit leading to the fuel pump can generate noise due to expansion of fuel when the valve therebetween opens. Since the conduit is essentially in a vacuum, the pressure differential across the valve is higher than the pressure within the reservoir.
There is thus a need for a fuel delivery system in which flow of fuel from the reservoir to the fuel pump is triggered in response to fuel level within the tank, but which is not dependent upon fuel pressure within the reservoir.